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There is now official confirmation that the unlawful interception of Kim Dotcom's communications carried on two months longer than anyone previously admitted.

But still no one is talking about it.

The confirmation came from police who investigated the Government Communications Security Bureau over the illegal spying.

The GCSB surveillance operation was carried out in support of the police raid on Dotcom's mansion on January 20, 2012, to assist the FBI takedown of the internet entrepreneur's Megaupload business over copyright breaches.

A High Court judgment released last week revealed the unlawful spying by the GCSB on Dotcom and others carried on two months after the arrests were made.

The judgment stated: "The Government Communications Security Bureau (GCSB) has admitted unlawfully intercepting private communications of Kim and Mona Dotcom (the Dotcoms) and Bram van der Kolk during the period from 16 December 2011 to 22 March 2012."

There had been speculation the judge had simply got the date wrong but a police statement that detectives were aware of the dates during the 2012/2013 investigation has put an end to that.

The extended date completely blows out accepted truth around the unlawful spying, with GCSB staff testifying in sworn affidavits to the High Court and Court of Appeal that it ended on January 20, 2012.

The GCSB, its minister Chris Finlayson and Prime Minister Bill English have refused to comment on the extended date for the end of the spying operation, which is contrary to previous sworn testimony in the High Court and the Court of Appeal.

All have said ongoing court action prevents them from doing so, with English personally named in Dotcom's damages claim for compensation over the unlawful spying.

The revelation of the unlawful spying was the catalyst for huge reform at the GCSB, which is New Zealand's link to a five-country spying network including Australia, Canada, the United States and United Kingdom.

Kim Dotcom making submissions to the Intelligence and Security select committee hearing into the GCSB in July 2013.

It also led to a police inquiry called Operation Grey which saw GCSB staff investigated over the illegal surveillance.

It found that there was no case for prosecution because there was no intent to break the law - the GCSB staff believed Dotcom and others were legally able to the spied on.

The NZ Herald asked police whether a new investigation would be able to be carried out.

A spokesman for police said: "We've checked the file and can confirm that the dates you've highlighted were known to the Operation Grey team. They were considered as part of the investigation and decision-making about the outcome."

The complaint was lodged by then-Green Party co-leader Russel Norman in September 2012, the same month Key apologised.

It was widened in April 2013 when an inquiry into the GCSB by then-Cabinet Secretary Rebecca Kitteridge (now NZ Security Intelligence Service) found 88 individuals had been illegally spied on by the GCSB between 2003 and 2012.

The police investigation was finished in August 2013 and a review supporting its conclusions was published by the Independent Police Conduct Agency in July 2014.

None of the material released publicly has extended the surveillance date with the exception of a statement of claim in Dotcom's claim for damages - dated April 2013 - saying the GCSB had admitted some surveillance continued until January 30 2012 because there were automated systems which couldn't be turned off.

A spokesman for the GCSB said: "As previously explained GCSB is not able to make comment on Justice Gilbert's decision as Mr Dotcom has indicated he will appeal it.

"As is evident from the judgment, the different dates have been known to the Court and Mr Dotcom's lawyers for some time."

The illegal surveillance not only saw upheaval at the GCSB but also won an apology for Dotcom and others from then-prime minister Sir John Key.

Prime Minister John Key days before apologising to Kim Dotcom over illegal GCSB spying.

It is unknown whether he - or English - were ever told the surveillance ended two months later than it was claimed at the time the apology was made.

It was also saw an inquiry by the Inspector-General of Intelligence and Security.

There is no detail in the Inspector-General's report suggesting an end-date of the surveillance, but the inquiry at the time was based on different information it could require a fresh investigation.

Kitteridge's report found the spies mistakenly believed they could carry out surveillance on New Zealand "residents" but not "permanent residents", even though the legislation offered the same protection to both.

No one had properly checked Dotcom's immigration status at the time surveillance started. A red flag was raised on February 20, 2012, and immigration records double-checked.

On February 27 2012 the GCSB's legal advisor, Hugh Wolfensohn, eased concerns saying - wrongly - that the bureau had complied with the law.